Coiled tubing well intervention has been known in the oil production industry for many years. A great length, often exceeding 15,000 ft, of small diameter, typically 1.5 in, steel tubing is handled by coiling on a large reel, which explains the name of coiled tubing. The tubing reel is not appropriate as a winch drum, since the stresses involved in using it so would destroy the tubing. The accepted solution in the industry is to pull tubing from the reel, as it is required and pass the tubing around a curved guide arch, or ‘gooseneck’, so that it lies on a common vertical axis with the wellbore. To control passage of tubing into and out of the wellbore, a device called a coiled tubing injector head is temporarily mounted on the wellhead, beneath the guide arch. By use of the injector head, the tubing weight and payload is taken from approximately straight tubing at the wellhead, leaving only a small tension necessary for tidy coiling to the tubing reel. Coiled tubing is externally flush and is ideal for insertion through a pressure retaining seal, or stuffing box, into a live well, that is one with wellhead pressure that would eject fluids if not sealed. Typically a coiled tubing injector head needs to be able to lift, or pull, 40,000 lbs as tubing weight and payload when deep in the well. It also has to be able to push, or snub, 20,000 lbs to overcome stuffing box friction and wellhead pressure at the beginning and end of a trip. The coiling tension is controlled by a tubing reel drive system and remains approximately constant no matter if the injector head is running tubing into or out of the well, or if it is pulling or snubbing. The coiling tension is insignificant by comparison to tubing weight and payload carried by the tubing in the wellbore and is no danger to the integrity of the tubing. The tubing is typically run to a great depth in the well and then cycled repetitively over a shorter distance in order to place chemical treatments or to operate tools to rectify or enhance the wellbore. It is by careful control of the injector head that the coiled tubing operator manipulates the tubing depth and speed to perform the programmed tasks. In order that the injector head may manipulate the tubing, it has to grip the tubing and then, concurrently, move the means of gripping so as to move the tubing within the wellbore. Although other methods of achieving this aim are known, it is the solution of a plurality of chain loops which is relevant to the present invention.
Referring to FIGS. 1 and 2 of the accompanying drawings, the chain loops 1, which are closed or endless, are moved by driveshafts 3 via mounted sprockets 4, engaging with roller chain links 5, which form part of the total chain loop assembly. A length of each loop, adjacent to the other chain loop over an essentially straight and parallel length, is forced by some arrangement, for example the hydraulically motivated roller and link assembly 6, into vigourous frictional engagement with the tubing 2 so as to grip the tubing 2 firmly and prevent its slipping, uncontrolled, into the well. Numerous patents describe improvements to the structure and mechanism of such injector heads. U.S. Pat. No. 4,585,061 describes an improved load-bearing structure for such a machine and U.S. Pat. No. 5,188,174 an improved mechanism for forcing the chain lengths into tighter frictional engagement with the tubing, without the tubing becoming overstressed towards the bottom of the chain run. U.S. Pat. No. 5,188,174 also discloses improvements to the chain loop structure, as illustrated in FIG. 2, in which the pins linking the roller chain to the gripper blocks 7 are separately removable for each of the two roller chains 5 in the chain loop assembly. Furthermore, considerable prior art exists concerning only the details of the chain loops. U.S. Pat. No. 5,853,118 describes an improved surface geometry for the gripper block with which to contact the tubing, as illustrated as feature 9 in FIG. 3 of the accompanying drawings, where the tubing gripping surface is shaped to fit tubing of a range of sizes by means of a vee shaped groove. The vee shaped groove is in contrast to the semi-circular cusp-shaped groove 8, as illustrated in FIG. 2, which is seen in many designs. U.S. Pat. No. 5,853,118 also discloses a simplification to the chain assembly where each gripper block is secured to the roller chains 5 by the use of a single through pin per block 10 rather than by the traditional two pins per block. U.S. Pat. No. 6,173,769, describes a means to change quickly the pipe contact elements, or ‘gripper blocks’, without complete disassembly of the many chain components, as illustrated in FIG. 4 of the accompanying drawings. A carrier element 11 is provided between two roller chains 5 into which a gripper block 12, appropriate for the chosen pipe size, may be releasably installed.
All of these prior art proposals rely on roller chains 5 and matching sprocket forms 4 as the means of transmitting drive from the driving shafts 3 to the chain loop assemblies 1. Roller chain is inexpensive, readily available and very strong, yet its incorporation into the chain loop assemblies results in a weight and maintenance burden, since the prior art assemblies all comprise the many elements of two heavy duty roller chains, plus the gripper blocks which contact the tubing and sometimes their separable carriers 11 too.
Accordingly, it is among the objectives of embodiments of the present invention to provide an improved chain assembly that is at least as strong as conventional arrangements, but simpler, lighter and easier to maintain, whilst retaining the advantage of the ability to provide an embodiment where one might quickly replace the gripper blocks.
Prior art arrangements also rely on one or more hydraulic motors to move the chain loop assemblies 1. Historically, hydraulic motors have been a sensible choice, combining all the virtues of mechanical simplicity, high power density, high starting torque, safety in a hazardous environment and simple control systems. However, there also exist a number of disadvantages, namely the requirement for bulky drive hoses, mechanical inefficiency leading to heat dissipation problems and limited control flexibility, especially at low constant speeds.
There are numerous prior art proposals which seek to minimize the disadvantages of hydraulic drive, leading to solutions employing any combination of hydraulic motors from one high-speed motor as disclosed in U.S. Pat. No. 6,059,029 and embodied in the Hydra Rig 580 product to four low-speed motors as deployed in the supplanted Hydra Rig 480 product.
It is among the objectives of embodiments of the present invention to provide an improved coiled tubing injector head that resolves the limitations of traditional hydraulic drive systems.
The prior art proposals further rely on a single pair of opposing chain loops. The gripping force applied to two opposing chain loops is fundamentally disposed to squeeze the tubing out of shape, a most undesirable consequence. In recognition of this, considerable prior art exists concerning the form of the gripper blocks which contact the tubing. It has been considered by many practitioners that a suitable gripper block form, which supports the tubing at positions around its circumference, would be a palliative to the crushing force applied by the chain loading assemblies. For example U.S. Pat. No. 5,188,174 talks of gripper blocks as having “(an) arcuate recess of the front tubing contact surface (which) is normally the same radius as that of the tubing” and this is illustrated as groove 8 in FIG. 2 of the accompanying drawings. The limitation of such a semi-circular cusp-shaped block is better understood when it is realized that coiled tubing varies slightly in diameter, both from manufacture and in normal use. When coiled tubing is moved into and out of a well, it commonly contains significant internal pressure. The tubing's bending radii, both at the tubing reel and at the wellhead guide arch, are sufficient to yield the tubing material. The combination of internal pressure with the yielding caused by bending is such that the tubing grows slightly but permanently in diameter. To allow both for this and for manufacturing tolerance, a semi-circular cusp-shaped block must necessarily be made slightly larger than the nominal diameter of the tubing. Therefore in many cases it will only make a single but soft line-contact with the tubing and the block will provide no lateral support until the tubing is significantly deformed. U.S. Pat. No. 5,853,118 describes an alternate approach for the gripper block with which to contact the tubing, as illustrated in FIG. 3 of the accompanying drawings, where the tubing gripping surface is shaped to fit tubing of a range of sizes by means of a vee shaped groove 9. The four line-contacts thus produced by the pair of chains are more equally disposed around the tubing than the traditional two line-contacts from cusp-shaped blocks, but with the disadvantage that the line-contacts are hard, being against a straight edge and not softened by the co-operating curve of a cusp. U.S. Pat. No. 5,309,990 reveals a composite block in which an elastomeric layer is used to support each pair of a quadrant of cusp-shaped gripping elements. As illustrated in FIG. 5 of the accompanying drawings, the compliance of the elastomer 13, in conjunction with the geometry of the underlying metal support 14, allows the gripping elements 15 to adjust to slightly differing pipe diameters, so providing a good soft line-contact at four equally spaced positions around the tubing. However, the resulting chain loop assemblies are complicated and expensive to maintain, since the elastomeric elements are severely stressed and often suffer only a short service life.
Accordingly, it is among the objectives of embodiments of the aspects of the present invention to provide an improved coiled tubing injector head which, though based on the successful and tested concept of a plurality of chain loops, addresses the issue of preventing the crushing force, necessary to grip the pipe, from deforming the tubing.